1. Field of the Invention
The present invention relates to a solid state imaging device and particularly, to an MOS-type solid state imaging device having an MOS transistor (MOS referred to by this specification is a general term for a conductive layer/an insulating film/semiconductor structure) and its manufacturing method.
2. Description of the Related Art
FIG. 1 shows a block diagram of an essential portion of a MOS-type solid state imaging device with a so-called FD (Floating Diffusion)-type arrangement. The arrangement of the solid state imaging device is such that a plurality of unit pixels 101 (only one unit pixel is shown in FIG. 1) are disposed in a plurality of rows and columns, that is, in the horizontal and vertical directions, and each unit pixel 101 has a sensor unit consisting of a photoelectric conversion element by a photo-diode 102 and in which a signal electric charge obtained by the sensor unit is read out by an FD read-out MOS transistor 103 and the signal electric charge is amplified to a signal voltage or signal electric current by an FD amplifying MOS transistor 104 in each unit pixel.
In the arrangement of FIG. 1, it is a case of an arrangement in which the signal amplification is carried out in each unit pixel 101, but as the block diagram of an essential portion in FIG. 2 shows, a so-called column amplifier-type solid state imaging device in which an amplifier is disposed at, for example, every common column can be made.
In the column amplifier-type solid state imaging device too, there is provided an arrangement such that a plurality of unit pixels 201 (also in FIG. 2, one unit pixel is only shown) are respectively disposed in a plurality of rows and columns, that is, in the horizontal and vertical directions, and each unit pixel 201 has a photo-diode 202 as the photoelectric conversion element in its sensor unit, and in which an MOS transistor 203 for reading out a signal electric charge accumulated in the photo-diode 202 and a selecting MOS transistor 204 for reading out the signal electric charge to a vertical signal line 208 are formed and a column amplifier 205 is disposed at every vertical signal line 208.
The photoelectric conversion element, that is, the photo-diode and the MOS transistor which carries out read-out of the electric charge therefrom at the sensor unit in each of the unit pixels 101 and 201 of these MOS-type solid state imaging devices are made a complex arrangement in which one semiconductor region constituting the photo-diode, for example, a cathode region is made to serve as a source region of the MOS transistor.
FIG. 6 shows a schematic plan pattern view of a sensor unit S and a forming portion of the MOS transistor which reads out the signal electric charge therefrom.
The sensor unit S is formed with a photoelectric conversion region 1 formed to constitute the photo-diode. The MOS transistor (MOS) is comprised of the photoelectric conversion region 1 as its source, a semiconductor region 2 as its drain, which is formed with a predetermined distance, that is, a distance corresponding to a channel length away from the photoelectric conversion region and a gate electrode 3 formed between them through a gate insulating film (not shown).
The sensor unit S is formed in one corner of, for example, the unit pixels 101 and 202 respectively explained in connection with FIG. 1 and FIG. 2 and the gate electrode 3 is formed to be displaced toward other circuit device forming portion side in the unit pixel.
The above-mentioned MOS-type imaging device has, because of being comprised of the MOS transistor, an advantage of fundamentally reducing power consumption in comparison with, for example, a CCD (charge-coupled device)-type imaging device, but there have occurred some problems in the fact that a signal electric charge is completely read out from the above-mentioned sensor unit at a low read-out voltage by the read-out transistor.